Method and material for coating metal surfaces



Patented Apr. 11, 1944 oer! METHOD AND MATERIAL FOR COATING METALSURFACES Noble 1E. Hays-and George R. Hoover, Middletown, and Alva E.

Ohio, assignors Taylor, West Middletown, to The American Rolling MillCompany, Middletown, Ohio, a. corporation of Ohio No Drawing.Application August 2, 1940, Serial No. 350,076

Claims. 7

This invention relates to the coating of zinc surfaces with asubstantially insoluble coating of phosphates which renders saidsurfaces particularly advantageous for the reception. and retention ofpaints, enamels and the like.

The diificulties encountered in coating zinc surfaces, such as ongalvanized sheets, with paints, enamels, or other siccative coatings arewell known. Difiiculty is experienced in applyin the coating and thereis a tendency. for it to become brittle and short time.

Various surface treatments have been suggested to overcome thisdifliculty, the most successful of which is commonly known asBonderizing. While this name is not limited to the treatment of zinc,the solution commonly called by this name which has proved mostsuccessful for zinc is one in which the zinc surface is treated in asolution containing phosphoric acid, phosphates, nitrates and a smallamount of another metal which is commonly iron. A more detaileddescription of the chemical features of these processes is contained inother patents, as for example Nos. 1,869,121; 2,082,950; and 2,067,214.While coatings produced by this treatment have proved very successfulwith respect to their superior ability to receive and hold paint,fabripeel off in a comparatively 'cators of galvanized sheets haveexperienced considerable difficulty in spot welding material which hasbeen given this treatment. Owners of large and powerful spot weldingmachines found that they could not regularly make successful welds onthis material in the cheap and rapid manner demanded by high speedcompetitive production. Smaller machines frequently could not makesatisfactory welds at all.

A primary object of our invention, therefore, is to provide a processfor the production of phosphate coatings of the Bonderized type on zincsurfaces, and in particularcn galvanized steel and iron sheets, whichwhile retaining all of the advantages of the previously used coatingscan, in addition, be readily spot welded.

In general, this object of our invention is achieved through theapplication of our discovery that there is a maximum weight of coatingwhich can be readily spot welded. From this follows another object ofour invention, which includes the provision of a method for producingcoatings of this character, and a coating composition by which thedesired results may be achieved.

As the result of extensive investigation we have discovered the relativeproportions of the various ingredients, and the conditions of treatmentin the treating bath which will consistently produce coatings below thismaximum value, but which will still be above the minimum weight ofcoating necessary to giv the desired paint service characteristics.

Our invention makes possible the application of such coated materials,having the combined advantages of the corrosion resistance of zinc andthe superior paintability and paint holding properties resulting fromthe coating, to many uses hitherto impossible because of theimpossibility of satisfactorily spot welding such materials.

By controlling the treatment in the manner to be described we limit theweight of coating to 0.030 oz. per sq. ft., which we have discovered tobe the practical maximum which can be spot welded satisfactorily on goodcommercial equipment. We prefer, however, to maintain the coat ingweight lower than this value and even approaching 0.015 02. per sq. ft.in order to make the material readily spot weldable even on somewhatinferior equipment. Below 0.015 oz. the paint preserving properties ofthe coating begin to be impaired. The range 0.030 to 0.015 issatisfactory as a good combination of these two properties, and inpractice we prefer to operate at around 0.025 oz. per sq. ft.- Thecoating weights given herein include the weight of the coating on bothsides of one square foot of material.

Attempts to obtain light coatings within this range by merely dilutingthe commonly used solutions, shortening the time of treatment, orlowering the temperature of treatment have not been productive ofsatisfactory results. The coatings resulting from the practice of suchexpedients are of non-uniform thickness over the sheet, are poorlyadherent and have otherwise unsatisfactory physical properties.

We have found it possible to produce consistently a light weight coatingwithin the limits of this range on conventional by using a solutionwithin the following composition ranges:

The acid concentration in "points we arrived at by titrating a 10 ml.the solution with 0.1 normal sodium is a meassample of hydroxide.

galvanized coatings changes in the zinc Nitrate per cent 0.68 to 1.08Free acid points 2.0 to 3.4 Total acid do 18.0 to 24.6 Zinc per cent0.38 to 0.65 Iron (ferrous) do 0.14 to 0.20 A particular solutioncomposition within these ranges which we have found to resultapproximately in our preferred weight of coating of around 0.025 oz. persq. ft. on conventional galvanized coatings is as follows:

Nitrate per cent 0.93 Free acid points 2.4 Total acid do 22.1 Zinc percent 0.45 Iron (ferrous) do 0.15-

In making up the original solutions, a convenient method of arriving atthis specified composition is by the addition of appropriate amounts ofphosphoric acid, zinc nitrate, and metallic iron to water; or the ironmay be added as ferrous phosphate. It will be within the skill of achemist to arrive at these compositions, as guided by these titrationprocedures.

As a guide to the maintenance of the bath at the proper compositionduring the passage of a large amount of material through it, it will befound that the addition of phosphoric acid will raise both the free andthe total acid concentrations. The addition of zinc nitrate willincrease the concentrations of zinc, nitrate and total acid. Aconvenient means of increasing the iron concentration is the addition ofiron in finely divided form such as steel wool, which'is soluble in thebath, or ferrous phosphate.

By way of comparison, the solution most commonly used before ourdiscovery, and which was productive of coatings too heavy to spot weldsatisfactorily, was approximately as follows:

Nitrate per cent 0.5 to 0.6 Free acid points 4 to6 Total acid do 30 Zincper cent 0.4

Iron (ferrous) do 0.25

The important change, which made our controlled light weight of coatingpossible, resides in the increased nitrate concentration, together withthe reduction in both free and total acids. The and iron concentrationsare only incidental since they'are of practically no significance inaffecting the weight or properties of the coating with the exception ofappearance: i. e., the higher the iron concentration, the darker thecolor of the coating; the higher the zinc concentration, the lighter thecolor of the coating.

The time and temperature of treatment in our new bath must be controlledto give a uniform coating with the proper adherence and surface texture.A treatment of around 40 to 60 seconds at approximately 135 F. ispreferred, although treatments up to 120 seconds have been used withoutexceeding the desired coating weight, since longer treatment does notincrease the weight of coating very rapidly. Treatments much less thanapproximately seconds will result in an uneven coating. Temperaturesbetween 115 and 145 F. are satisfactory but temperatures greatly inexcess of this range, such as above 160 F. should be avoided on accountof the formation of an excessive amount of sludge in the bath. Atsomewhat lower temperatures, such as below 100 F. a heavier coating ofunsatisfactory texture is produced.

Additional treatments preceding or following the coating step describedhave been used in the case of the previous process for producing heavycoatings, and although forming no part of this invention, are equallyapplicable to our new process for the production of light coatings; forexample, a chromic acid rinse A machine which we have adapted to theapplication of our solutions is described in Finkbone et al., Patent No.2,175,599, and the rubber wetting rolls described therein areparticularly useful in assisting in the production of auniform coating.

While we have used the term sheet herein, this is not to be consideredas a word of limitation, and it will be understood that wherever thisword has been used it includes the word strip.

While we have mentioned herein certain compositions, concentrations,weights, temperatures and times, which we consider preferable, it willbe clear that these are exemplary, and that we are not limited as tothese factors otherwise than as pointed out in the claims which follow.

Having now fully described our invention, what we claim as new, anddesire to secure by Letters Patent is:

1. A coating bath containing phosphoric acid for producing controlledlight weight phosphate coatingsonzinc surfaces in which nitrate ispresent within the range 0.65 to 1.10% and in which the free acid doesnot exceed 4.0 points and the total acid does not exceed 25.0 points.

2. A coating bath containing phosphoric acid for producing controlledlight weight phosphate coatings on zinc surfaces in which nitrates arepresent within the range 0.65 to 1.10%, zinc is present in the range0.35 to 0.70%, iron (ferrous) is present in the range 0.10 to 0.25%, andin which the concentration of free acid is within the range 1.5 to 4.0points, and in which the concentration of total acid is within the range16.0 to 25.0 points.

3. A coating bath containing phosphoric acid for producing controlledlight weight phosphate coatings on zinc surfaces in which nitrates arepresent within the range 0.68 to 1.08%, .zinc is present in the range0.38 to 0.65%, iron (ferrous) is present in the range 0.14 to 0.20%, andin which the concentration of free acid is within the range 2.0, to 3.4points, and in which the concentration of total acid is within the range18.0 to 24.6 points.

4. A coating bath containing phosphoric acid forproducingcontrolled-light weight phosphate coatings on zinc surfaces inwhich nitrates are present in an amount substantially 0.93%, zinc ispresent in an amount substantially 0.45%, iron (ferrous) is present inan amount substantially 0.15%, and in which the concentration of freeacid is substantially 2.4 points and the concentration of total acid issubstantially 22.1 points.

5. A process of producing controlled light weight phosphate coatings onzinc surfaces which includes the steps of treating said metal surfacesin a bath according to claim 1 for not less than 20 seconds and not morethan 120 onds at a temperature not less than 100 F. and r not more than160 F.

onds at-approximately 135 F 9. A process range of 0.35 to 0.70%, ironwithin the range of 0.10 to 0.25%, and showing upon titration with 0.1 afree acid content (as indicated by methyl 15. As an article ofmanufacture, a galvanized metal sheet NOBLE E. HAYS. GEORGE R. HOOVER.ALVA E. TAYLOR.

